[PATCH 3.2 75/77] af_unix: fix a fatal race with bit fields

Ben Hutchings
2015-12-24T15:43:51Z

From: Eric Dumazet <eric.dumazet@gmail.com>

commit 60bc851ae59bfe99be6ee89d6bc50008c85ec75d upstream.

Using bit fields is dangerous on ppc64/sparc64, as the compiler [1]
uses 64bit instructions to manipulate them.
If the 64bit word includes any atomic_t or spinlock_t, we can lose
critical concurrent changes.

This is happening in af_unix, where unix_sk(sk)->gc_candidate/
gc_maybe_cycle/lock share the same 64bit word.

This leads to fatal deadlock, as one/several cpus spin forever
on a spinlock that will never be available again.

A safer way would be to use a long to store flags.
This way we are sure compiler/arch wont do bad things.

As we own unix_gc_lock spinlock when clearing or setting bits,
we can use the non atomic __set_bit()/__clear_bit().

recursion_level can share the same 64bit location with the spinlock,
as it is set only with this spinlock held.

[1] bug fixed in gcc-4.8.0 :
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080

Reported-by: Ambrose Feinstein <ambrose@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
include/net/af_unix.h | 5 +++--
net/unix/garbage.c | 12 ++++++------
2 files changed, 9 insertions(+), 8 deletions(-)

--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -54,9 +54,10 @@ struct unix_sock {
struct list_head link;
atomic_long_t inflight;
spinlock_t lock;
- unsigned int gc_candidate : 1;
- unsigned int gc_maybe_cycle : 1;
unsigned char recursion_level;
+ unsigned long gc_flags;
+#define UNIX_GC_CANDIDATE 0
+#define UNIX_GC_MAYBE_CYCLE 1
struct socket_wq peer_wq;
wait_queue_t peer_wake;
};
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -185,7 +185,7 @@ static void scan_inflight(struct sock *x
* have been added to the queues after
* starting the garbage collection
*/
- if (u->gc_candidate) {
+ if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
hit = true;
func(u);
}
@@ -254,7 +254,7 @@ static void inc_inflight_move_tail(struc
* of the list, so that it's checked even if it was already
* passed over
*/
- if (u->gc_maybe_cycle)
+ if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
list_move_tail(&u->link, &gc_candidates);
}

@@ -315,8 +315,8 @@ void unix_gc(void)
BUG_ON(total_refs < inflight_refs);
if (total_refs == inflight_refs) {
list_move_tail(&u->link, &gc_candidates);
- u->gc_candidate = 1;
- u->gc_maybe_cycle = 1;
+ __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
+ __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
}
}

@@ -344,7 +344,7 @@ void unix_gc(void)

if (atomic_long_read(&u->inflight) > 0) {
list_move_tail(&u->link, &not_cycle_list);
- u->gc_maybe_cycle = 0;
+ __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
scan_children(&u->sk, inc_inflight_move_tail, NULL);
}
}
@@ -356,7 +356,7 @@ void unix_gc(void)
*/
while (!list_empty(&not_cycle_list)) {
u = list_entry(not_cycle_list.next, struct unix_sock, link);
- u->gc_candidate = 0;
+ __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
list_move_tail(&u->link, &gc_inflight_list);
}